Colorful parts at lower cost.

Try Screen-Printed In-Mold Decoration

What if you could make a product in a range of colors without having to purchase numerous precolored resins or color concentrates - and without painting? How about producing multi-tinted clear parts such as lenses without the expense of multi-component injection molding? These are some of the possibilities offered by in-mold decoration with preprinted and preformed film inserts, according to GE Plastics Structured Products, Pittsfield, Mass. GE has been working on in-mold decoration (IMD) technology at a new screen-printing lab at its Plastics Processing Development Center and recently published a 25-page technical guide, In-Mold Films: A Guide for Designing, Forming and Molding with Screen-Printed Lexan Films.

GE has explored both new applications for IMD and ways to optimize a process that involves screen printing on its Lexan polycarbonate films, thermo-forming the printed film, and then inserting the preform into a mold cavity prior to injection molding the finished part. The decoration is now molded into the part.

While IMD is not a new technology, industry manager David Reis says it has only recently been used to create 3D products with complex surfaces. "We have developed a number of proprietary processes that deliver a new creative freedom to industrial designers as well as considerable cost savings on the production floor," he says.

Typical IMD applications include decorative nameplates, back-lit automotive instrument-panel displays, and buttons, appliance appliques, and lenses for pagers and cellular phones.

SECOND-SURFACE IMD

According to Reis, the maximum benefit of IMD is achieved with "second-surface" decorating. This involves printing on the back side of the clear PC film - the side that will face away from the injection mold surface. The decoration is thereby embedded in the part and is permanently protected from abrasion and chemical attack.

IMD with second-surface printing offers a number of benefits, explains Reis. "First, it enables clear product differentiation, now critical for success in the marketplace. Second-surface IMD, particularly when implemented on Lexan films, protects graphics and integrates them into the final molded part." It also allows for the production of many different graphics through use of different inserts on one tool. IMD eliminates secondary operations on molded parts and thus can reduce total system cost in many applications, Reis says. Cost savings relative to traditional decorating processes, such as heat-transfer labels, reportedly can be up to 20-30%, GE says.

As explained in GE's new guide, flat-film parts with first-surface graphics are the simplest to produce with IMD technology. Lexan film is first screen printed, die-cut to size, and then positioned and secured in an injection molding tool designed for IMD. Resin is then injected onto the film, and a finished part is ejected.

Flat-film parts with second-surface graphics are similar, but the graphics are now printed on the back of the film. The key difference is that second-surface IMD requires more robust ink systems to withstand direct contact with the hot melt, and the gating and resin choices for the molding process require special consideration so as not to disturb the printed decoration.

Curved film parts with first-surface graphics are a bit more challenging, as the film must be pre-shaped by hot or cold forming. Curved film parts with second-surface graphics are the most challenging to manufacture as they combine the complexity of forming the film with the difficulties of injecting resin onto the surface containing the printed graphics.

MATERIAL SELECTION IS KEY

GE's new guide advises that film choice is critical to the process and should depend on the application. Minimum film thicknesses of 0.0070.010 in. are typically suggested for IMD. GE recommends an uncoated Lexan film for decorating 3D or textured parts, such as automotive and telecommunications components. However, if a part requires chemical or abrasion resistance, a coated film such as Lexan HP is recommended, but the part can only be fiat or two-dimensional. Applications may include appliance or some telecommunications parts. GE is developing new formable laminates and coated films for 3D IMD applications that require even more chemical and abrasion resistance.

IMD film inserts can be backed up in the mold with polycarbonate resin or PC alloys such as GE's Cycoloy PC/ABS or Xenoy PC/PBT. Valox PBT resins also have acceptable in-mold adhesion to Lexan films, GE reports.

The new guide also covers ink selection, which is based on the part specifications and the end-use application. Other aspects of the IMD process that are covered in the guide are design, tooling, and processing considerations for printing, forming, trimming, and molding. The free guide may be obtained by calling (800) 845-600 or GE's fax-back service at [phone].